1. Field of the Invention
The present invention relates in general to improvements in a grindstone or grinding wheel to be used in a thru-feed centerless grinding operation.
2. Discussion of the Related Art
As a type of industrial grindstone, there is known a grinding wheel which is to be brought into sliding contact with a workpiece while being rotated about its axis, so that a surface of the workpiece is ground by an abrasive layer which is provided by an outer circumferential surface of the grinding wheel. The abrasive layer has a longer service life where the abrasive layer is formed of so-called xe2x80x9csuper abrasive grainsxe2x80x9d such as diamond abrasive grains and CBN (cubic boron nitrides) abrasive grains, than where the abrasive layer is formed of standard abrasive grains such as alumina abrasive grains and silicone carbide abrasive grains. Where the abrasive layer is formed of the super abrasive grains, the abrasive layer has a relatively small thickness, in general, due to a relative expensiveness of the super abrasive grains. In recent years, the grinding wheel having the abrasive layer formed of the super abrasive grains is widely used, thereby contributing to an unmanned or automated grinding operation in a machining industry. Therefore, the grinding wheel of the super abrasive grains is employed in various fields of industry, and is an object of further research and development for further improvement of its grinding performance.
As an example of a grinding operation in which the above-described grinding wheel is employed, there is known a centerless grinding operation in which a cylindrical workpiece is not supported on its centers but rather by a work rest blade, a regulating wheel and the grinding wheel, so that the cylindrical workpiece is ground mainly at its outer circumferential surface by the grinding wheel. FIG. 1 is a view illustrating a thru-feed centerless grinding operation in which cylindrical workpieces 14 are successively fed to a grinding zone in which each of the workpieces 14 is actually ground at its outer circumferential surface by the grinding wheel in the form of a segment-chip-type grinding wheel 10. The workpieces 14, which are disposed on the work rest blade 18 (which is positioned between the grinding wheel 10 and the regulating wheel 12) and are guided by work rest guides 16, are successively moved or fed in a predetermined feed direction, i.e., a longitudinal direction as indicated by the arrow, while being gripped by and between the grinding wheel 10 and the regulating wheel 12. In this instance, the regulating wheel 12 and the grinding wheel 10 are rotated in the same direction, namely, the clockwise direction as seen in the above-described feed direction. Described more specifically, the regulating wheel 12 is rotated for rotating workpieces 14 at a relatively low speed, while the grinding wheel 10 is rotated at a relatively high speed, whereby the outer circumferential surfaces of the workpieces 14 are grounded by an abrasive layer provided by an outer circumferential surface of the grinding wheel 10.
In the above-described thru-feed centerless grinding operation, the workpieces 14 are fed at a feed rate of, for example, about 5-10 m/min in the feed direction indicated by the arrow. The work rest guides 16 guiding the workpieces 14 are positioned in the upstream and downstream sides of the wheels 10, 12 as viewed in the feed direction. Each of the work rest guides 16 is not held in contact with the wheels 10, 12 but is necessarily spaced apart from the wheels 10, 12 as viewed in the feed direction. Due to the spacing region between an upstream side one of the work rest guides 16 and the wheels 10, 12, each workpiece 14 fed in the feed direction could be momentarily shaken or oscillated in a direction perpendicular to the feed direction, when the workpiece 14 is passing an entrance of the grinding zone, i.e., an upstream end portion of the grinding wheel 10. Upon initiation of contact of the workpiece 14 (at its forward end portion) with the grinding wheel 10, the grinding wheel 10 receives at its upstream end portion an impact or shock from the workpiece 14, thereby possibly causing a large amount of wear in the upstream end portion of the grinding wheel 10. Similarly, due to the spacing region between a downstream side one of the work rest guides 16 and the wheels 10, 12, each workpiece 14 could be oscillated when the workpiece 14 is passing an exit of the grinding zone, i.e., a downstream end portion of the grinding wheel 10. This shaking or oscillating motion of the workpiece 14 in the exit of the grinding zone is likely to cause a deterioration in a machining accuracy of the grinding operation.
The present invention was made in the light of the background art discussed above. It is therefore an object of the present invention to provide a cylindrical grindstone or grinding wheel which is capable of grinding a workpiece with a high degree of machining accuracy without suffering from a large amount of wear in its local portion. This object of the invention may be achieved according to any one of the first through tenth aspects of the invention which are described below.
The first aspect of this invention provides a grinding wheel comprising: a cylindrical main body having a grinding surface on an outer circumferential surface thereof; and a pair of synthetic resin layers disposed on respective axially opposite end faces of the cylindrical main body, each of the synthetic resin layers covering at least a radially outer end or peripheral portion of a corresponding one of the axially opposite end faces.
In the grinding wheel according to this first aspect of the invention, the synthetic resin layers are provided on the respective axially opposite end faces of the cylindrical main body such that each of the synthetic resin layers covers at least the radially outer end portion of the corresponding one of the axially opposite end faces. This arrangement is advantageous in the above-described thru-feed centerless grinding operation, because the synthetic resin layers serve to reduce the problematic oscillating motion of the workpiece upon its entrance into the grinding zone and also upon its exit from the grinding zone, thereby making it possible to grind the workpiece with a high degree of machining accuracy without suffering from a large amount of wear in a local portion of the grinding wheel.
According to the second aspect of the invention, in the grinding wheel defined in the first aspect of the invention, the cylindrical main body has an abrasive layer which constitutes a radially outermost layer thereof so that the abrasive layer provides the grinding surface.
According to the third aspect of the invention, in the grinding wheel defined in the second aspect of the invention, each of the synthetic resin layers has an elastic modulus lower than that of the abrasive layer.
According to the fourth aspect of the invention, in the grinding wheel defined in any one of the first through third aspects of the invention, the synthetic resin layers have respective elastic moduli which are different from each other.
In the thru-feed centerless grinding operation, a position of the regulating wheel relative to the grinding wheel may be adjusted such that a spacing distance between the two wheels is not constant as viewed in the feed direction. For example, the spacing distance may be larger in the entrance of the grinding zone, than in the exit of the grinding zone, so that the diameter of the workpiece is gradually reduced as the workpiece is fed through the grinding zone in the feed direction. In this case, the diameter of the workpiece is approximated to a target dimension by grinding the workpiece with the upstream end and intermediate portions of the grinding wheel, and then the diameter is reduced precisely to the target dimension by grinding the workpiece with the downstream end portion of the grinding wheel. In such a case, it is preferable that the grinding wheel is set on a grinding machine such that one of the synthetic resin layers having relatively low degree of elastic modulus is positioned in an upstream side of the other synthetic resin layer (having relatively high degree of elastic modulus) as viewed in the feed direction. In this preferable arrangement, owing to this setting of the grinding wheel on the grinding machine, the shaking or oscillating motion of the workpiece in the entrance of the grinding zone is reduced by the upstream-side synthetic resin layer having the relatively low degree of elastic modulus, and the machining accuracy of the workpiece is improved by the downstream-side synthetic resin layer having the relatively high degree of elastic modulus.
According to the fifth aspect of the invention, in the grinding wheel defined in any one of the first through fourth aspects of the invention, each of the synthetic resin layers contains a ceramic material as an aggregate thereof. In this arrangement, each of the synthetic resin layers can be given a desired degree of elastic modulus, by changing the content of the ceramic material.
According to the sixth aspect of the invention, in the grinding wheel defined in any one of the first through fifth aspects of the invention, each of the synthetic resin layers includes a phenol resin as a main component thereof. Since the phenol resin is of a synthetic resin material that is excellent in its heat resistance, elasticity and mechanical strength, it is possible to more effectively minimize an abnormal wear in a local portion of the abrasive layer, and further improve a machining accuracy in a grinding operation.
According to the seventh aspect of the invention, in the grinding wheel defined in any one of the first through sixth aspects of the invention, each of the synthetic resin layers has an elastic modulus of 300-6000 kg/cm2.
According to the eighth aspect of the invention, in the grinding wheel defined in any one of the first through seventh aspects of the invention, each of the synthetic resin layers is provided by an annular member having an outer circumferential surface which has an outside diameter equal to an outside diameter of the outer circumferential surface of the cylindrical main body and which is coaxial with the outer circumferential surface of the cylindrical main body.
According to the ninth aspect of the invention, in the grinding wheel defined in the eighth aspect of the invention, the cylindrical main body has, in respective axially opposite end portions thereof, small diameter portions each of which has an outside diameter equal to an inside diameter of a corresponding one of the synthetic resin layers, and which has an axial length equal to an axial length of the corresponding one of the synthetic resin layers, so that the synthetic resin layers are mounted on the respective axially opposite end portions of the cylindrical body.
According to the tenth aspect of the invention, in the grinding wheel defined in any one of the second through ninth aspects of the invention, the cylindrical body includes a cylindrical core body and a plurality of abrasive segment chips which are fixed to an outer circumferential surface of the cylindrical core body and which cooperate with each other to constitute the abrasive layer.